Piper/response validation

### What is the problem / feature ?

The library has no mechanism for validating an api response against a declared swagger schema.

### How did it get fixed / implemented ?

`PropertySerializer.save()` now returns a callable that can be used to validate a json response against its declared schema.

### How can someone test / see it ?

Not sure yet what the api is going to be and how this fits in with the overall swagger schema.


*Here is a cute animal picture for your troubles...*

![enhanced-19397-1394043219-2](https://cloud.githubusercontent.com/assets/824194/4712466/a3bf92a2-58ce-11e4-845b-f7361a4dd35e.jpg)